Abstract
Population replenishment through recruitment is an essential process for the long term viability of corals and their associated communities, particularly under increasing stresses that threaten their vitality. Although many researchers have identified specific factors that influence individual processes in the early life cycle of corals, few studies to date have attempted to determine the cumulative success of a cohort's progression through these stages in natural reef settings. Specifically, there is a paucity of knowledge regarding appropriate and realistic techniques to forecast the success of recruitment in natural settings, while taking into account both the individual and environmental factors that regulate these recruitment dynamics at local scales. Because of this need, the overall goals of this dissertation research were to (1) assess key life stage processes leading to recruitment - specifically, settlement and early post-settlement processes - for which previous knowledge was limited or absent; and (2) using this knowledge, develop a local-scale recruitment model that assessed the cumulative success of a cohort's progression through all the early life stages and identified those processes that had a strong relative influence on regulating recruitment dynamics. Focusing on the common western Atlantic brooding coral, Porites astreoides, this dissertation research was divided into three main sections to address the overall objectives: (1) identification and quantification of recruitment patterns in natural reef settings, in order to guide the development and testing of the recruitment model (Chapter 2); (2) assessment of the focal species' behaviors, survivorship rates, and factors affecting those rates during its progression through the primary early life stage processes (i.e., basic habitat preferences during the settlement stage, Chapter 3; early post-settlement survivorship, Chapters 4 and 5); and (3) development of a local recruitment model that accounted for the full complement of early life stage processes in a spatially-explicit simulation framework (Chapter 6). While unique study-specific insights were gained from each of the individual chapters, a few general insights emerged with respect to the overarching study objectives from this dissertation research. First, larval supply is a key driver for recruitment, where a high degree of larval loss, either through direct larval mortality or export from the reef, occurs prior to settlement on the substrate. Rates of loss were 96-99% in the model analyses, and as such represent the first major population bottleneck for this species and others with similar life histories. Compounding this larval loss is a second population bottleneck during the early post-settlement stage, where mortality was typically greater than 75% within the first week after settlement. Such high rates of loss have important implications for future population dynamics, as relatively minor changes to these rates of loss can have relatively strong influences on future dynamics. Second, habitat influences on recruitment were found to be relatively minimal when compared to high rates of mortality in both the larval supply and early post-settlement stages. Although the relative influence of habitat may be strong under unique situations where substantial space preemption limits settlement (e.g., high macroalgal cover, sedimentation, or adult coral cover), these effects may not be reflective of average systems. However, the influence of habitat may still be crucial for ensuring that the few individuals who survive the larval supply and the early post-settlement bottlenecks recruit into the future adult population, and these influences may interact with other density-dependent processes as adult cover increases. Overall, this research presents valuable and novel insights on a number of the under-studied early life stage processes. By identifying the key processes which regulate recruitment, this work highlights those stages whose responses to environmental change will have strong impacts on recruitment and subsequent population dynamics. In addition to the process-based insights gained on these dynamics, this work provides informative criteria for managers on the stages most responsive to conservation efforts aimed at promoting resilience and recovery.